The term “communication station” or “station” refers to a station comprising one or more transmitters or receivers, or a set of transmitters and receivers, including the auxiliary devices, which are required to provide a communication service in a given location.
A station placed on a high-altitude platform or HAPS, for high-altitude platform station refers to a station installed on an object placed at high altitude, for example above 18 km, and at a specified, nominal point that is fixed with respect to the Earth. A network gateway station refers to a ground station that is intended to provide a communication link with a station placed on a high-altitude platform and to control access to the core network.
FIG. 1 shows an example of a system for telecommunications via stations placed on stationary high-altitude platforms for broadband access to a terrestrial network. This system comprises a station placed on a stationary high-altitude platform SHA which communicates both with a core network RC via a network gateway station SP and with a set of user terminals TU_1, TU_2 that are dispersed throughout a coverage area ZC, to provide them with broadband access to the Internet or to other terrestrial communication networks. The stationary high-altitude platform SHA is provided with a communication payload or station comprising means for establishing bidirectional radio links 121, 122 with the mobile or fixed terminals TU_1, TU_2 and a bidirectional radio link 101 with the gateway station ST. The gateway station ST comprises means for establishing a communication link 103 for establishing access to the core network RC. The high-altitude platform is provided with propulsion means, for example propeller motors, for compensating for the strength of the winds and for remaining stationary in the sky from the point of view of the user terminals and of the gateway station. The coverage area ZC is the geographical area from which the stationary high-altitude platform is visible from the ground with a positive elevation angle, for example 5 degrees. The higher the flight altitude of the stationary high-altitude platform, the greater the radius of the coverage area. Thus, for example, a stationary high-altitude platform flying at an altitude of 20 km allows visibility with an elevation of 5 degrees in a coverage area with a radius of 194 km. The choice of flight altitude is relatively restricted by the presence of strong winds in the stratosphere. However, between the altitudes of 18 and 25 km, the stratospheric winds are gentler (typically lower than 20 m/s) and this range of altitudes is therefore chosen for positioning the stationary high-altitude platforms.
A system for telecommunications via stations placed on stationary high-altitude platforms has some drawbacks. A first drawback is the decreased size of the coverage area due to the flight altitude of the stationary high-altitude platforms being limited in practice to 20 km to take advantage of gentler winds. This limitation in the size of the coverage area decreases the number of users that are able to benefit from the access service and hence the profitability of the system.
A second drawback is the low elevation at which users on the edges of the coverage area view the stationary high-altitude platform in the sky. Thus, on the periphery of the coverage area, users are masked by hills, buildings or vegetation. There will therefore be “blind spots” in the coverage where the radio links with the terminals are blocked or attenuated by obstacles. Users fixed in blind spots or moving through blind spots will not be able to access the access service or will access it with poor quality of service.
The problem targeted by the present invention therefore consists in designing a system for telecommunications via stationary high-altitude platform stations having a coverage area that extends beyond the geographical area in which the user terminals have the high-altitude platform in their line of sight.
American patent U.S. Pat. No. 8,718,477 describes a telecommunications system based on a network of drifting balloons according to an ad-hoc mesh network principle.
Such a system has the advantage of providing a large coverage area but has the drawback of deploying a large number of balloons to provide lasting coverage, i.e. continuous coverage over time, for a given geographical area. Specifically, since a balloon is drifting by nature, it moves with the winds and its coverage area varies over time with this movement.
It is therefore desirable to provide a telecommunications system which retains some of the advantages of the systems using stationary high-altitude platforms while providing a large geographical coverage area which is more reliable and longer lasting than that provided by a network of drifting balloons.